Diamond is a preferred material for semiconductor devices because it has semiconductor properties that are better than traditionally used silicon (Si), germanium (Ge) or gallium arsenide (GaAs). Diamond provides a higher energy band gap, a higher breakdown voltage and a greater saturation velocity than these traditional semiconductor materials. These properties of diamond yield a substantial increase in projected cutoff frequency and maximum operating voltage compared to devices fabricated using Si, Ge, or GaAs. Si is typically not used at temperatures higher than about 200.degree. C. and GaAs is not typically used above 300.degree. C. These temperature limitations are caused, in part, because of the relatively small energy band gaps for Si (1.12 eV at ambient temperature) and GaAs (1.42 eV at ambient temperature). Diamond, in contrast, has a large band gap of 5.47 eV at ambient temperature, and is thermally stable up to about 1400.degree. C.
Diamond has the highest thermal conductivity of any solid at room temperature and exhibits good thermal conductivity over a wide temperature range. The high thermal conductivity of diamond may be advantageously used to remove waste heat from an integrated circuit, particularly as integration densities increase.
Because of the advantages of diamond as a material for semiconductor devices, there is at present an interest in the growth and processes for etching diamond. There has been particular interest in the growth and etching of single crystal diamond thin films. Diamond thin films have been successfully grown by a variety of low pressure and low temperature chemical vapor deposition (CVD) methods, such as filament assisted CVD, plasma CVD, and combustion flames. See Devaries, Ann. Rev. Mater. Sci. 17, 161 (1987); Masumoto et al, J. Mat. Sci. 18, 1823 (1988). In these methods, typically a mixture of hydrocarbons such as CH.sub.4 and C.sub.2 H.sub.2 and hydrogen (98-99.5% by volume) is passed over heated (700.degree.-1000.degree. C.) substrates such as silicon, nickel, copper, tungsten, and silicon carbide. During the process, both diamond and graphite are deposited. However, etching of graphite by hydrogen leaves diamond crystallites which grow further to form polycrystalline films.
Single crystal diamond thin films have been grown epitaxially on diamond substrates, where all three axes of the film are aligned with respect to the underlying substrate (See Geis et al, IEEE-Electron Device Lett. EDL-8, 341 (1987); M. W. Geis, Materials Res. Soc. Pro., 162, 16 (1990)). Homoepitaxial diamond layers can be grown by CVD onto a matrix of oriented diamond microcrystallites embedded in array of etch pits on a silicon substrate (See, Pryor et al., Materials Res. Soc. Pro. 242 (1992)). This method, however, is both costly and time-consuming, and the crystallites are only poorly oriented and have low angle grain boundaries. Moreover, there continues to be a need for thin large area sheets of diamond from bulk or homoepitaxial diamond crystals inasmuch as at present typical crystal diamond substrates have at most an area of several millimeter square.
Another difficulty in processing diamond substrates to be used in fabricating semiconductor devices is the ability to selectively etch the diamond substrate. It is known to etch graphites from diamond by immersion in hot chromic acid. Chromic acid is highly corrosive and toxic thus its large scale use is problematic. Similar to other chemical etchants, chromic acid has a limited useful lifetime and must be prepared fresh for each etch. Secondary acid washes are necessary for removal of chromium deposits from the surface of the sample. Chromic acid provides a relatively slow etch rate of about 0.1 to 0.4 .mu.m/hr.